Half-way along the main corridor was a large room, whose steel walls were masked by tapestries, where renters could examine their treasures on marble tables. It was empty when Simon went in. The patrol carefully closed the door on him, and then in a moment came back to say that Mr. Brown was not in his office, and had probably gone out to lunch, the hour being noon. 'Where did you see Mr. Hugo?' Simon asked, hurrying out of the room in a state of considerable agitation. 'I saw him just here, sir,' said the patrol, turning down a short side corridor--the grille was unfastened--and stopping before a door numbered thirty-nine. 'He was talking to Mr. Brown, and the door of the vault was open.' 'That must be Mr. Polycarp's vault,' Simon observed; and then he started, and put his ear against the door. 'Listen!' he exclaimed to the patrol. 'Can't you hear anything inside?' And the patrol also put his ear to the steel face of the door. 'I seem to hear a faint knocking, but it's that faint as you scarcely _can_ hear it. There! it's stopped.' 'He is inside,' Shawn whispered. 'Who's inside?' 'Mr. Hugo.' 'It's God help him, then,' said the patrol, 'if he's there long. There's no ventilation, Mr. Shawn. We'd better telephone for Mr. Polycarp. The other key will be in the key-safe. I can get it. But how do you make out, sir, that Mr. Hugo can be in there? The vault could only be locked by Mr. Polycarp and Mr. Brown together, and surely they couldn't both--' 'Mr. Polycarp left his keys behind by accident. He had gone before Mr. Hugo came down.' 'There's been no Mr. Polycarp here this morning,' said the patrol a minute later. 'I've looked at the signature-book. I thought it was queer I hadn't seen him. And, what's more, that isn't Mr. Polycarp's vault at all. Mr. Polycarp's vault is No. 37. This vault has been empty for several weeks.' 'Then you have both the keys?' Simon demanded quickly. 'No, sir. It's very strange. There's only one key of No. 39 in the key-safe, and it's the renter's key.' 'Then Mr. Brown must have the other.' 'I expect so. But he ought not to have. It's against rules,' said the patrol. 'I know where he takes his lunch. I'll send for him.' Simon put his ear again to the face of the door. The faint knocking had ceased, but after a few seconds it recommenced. 'And suppose you don't find Mr. Brown?' he queried, still listening. 'Then that vault can't be opened. But never you fear, Mr. Shawn. I'll have him here in three minutes. It's funny as he should have left anybody in there by accident--and Mr. Hugo of all people in this blessed world....' The patrol's accents died away as he passed down the main corridor.